The present invention relates to a novel method for manufacture of an improved cellular elastomeric polyurethane foam substantially spherical ball, the reaction mixture required by the method, and the product thereof. The present method is commercially expedient and economical. The product elastomeric polyurethane foam ball may be made into a size having excellent properties for use as a tennis ball core, such as, for example, weight, diameter, bound (or rebound), forward deformation (or deflection) and return deformation (or deflection). When elastomeric polyurethane foam balls of the present invention are used as cores for tennis balls, the tennis balls satisfy the strict specifications set by the International Tennis Federation (ITF) for tennis ball acceptability.
Conventional tennis balls are currently manufactured to be felt covered pressurized bladders of elastomeric material which is usually a composition of rubber or rubber-like material, or felt covered non-pressurized rubber or polyurethane foam cores. The pressurized core balls are filled with gas, usually air, at a required pressure, such as for example about 0.7 kg/cm2, and lose pressure over time of play and storage. The non-pressurized core balls are manufactured by commercially cumbersome, multi-step processes to give them playability and usually play with a different feel or fail to meet the rigid specifications set by the ITF. Due to the importance of tennis balls meeting the specifications set by the ITF, such as, for example, weight, diameter, bound, forward deformation and return deformation, researchers have tried to achieve these properties by a variety of different methods. Many of such methods and the products comprising the results of such methods, e.g. tennis balls, have troublesome shortcomings or deficiencies which create problems, such as, for example, the ball having insufficient bound making such a ball unacceptable for use in ITF sanctioned play, or having an unusual feel to the player.
Both pressurized and non-pressurized tennis balls can be made to meet specifications of the ITF, such as diameter, weight, bound and forward and return deformation. However, a tennis ball also ideally exhibits long play life and consistency in play behavior over time. Pressurized tennis balls exhibit a relatively short life due to gas permeating through the rubber or rubber-like material defining the core. Many current pressureless tennis balls exhibit longer life than pressurized balls, but are difficult to manufacture with consistent play behavior.
U.S. Pat. No. 6,106,419 relates to a non-pressurized ball, such as a non-pressurized tennis ball. The ball of this patent has a wall of elastomeric material defining a cavity containing a filling which comprises a plurality of substantially closed cells. The ball of this patent is manufactured by inserting a preformed unexpanded material into at least one of a pair of hemispherical half shells; bonding the two half shells together to form a cavity; and expanding the material to form a filling of polymeric microspheres which substantially fill the cavity. More specifically, the manufacturing method comprises a first step of mixing elastomeric materials with different chemical ingredients. The mixture is milled to a smooth consistency and fed into an extruder which forms the mixture into preformed structures. The elastomeric preformed structures are then placed into a multicavity mold. Under pressure and heat, the preformed structures are formed into hemispheres, each of which is one half of a tennis ball core. These halves are edge ground and the edges are coated with adhesive in a double-carousel assembly unit. Pellets containing pure or matrix-bound unexpanded microspheres are added to the half shells on one carousel. The half shells meet, one from each of the carousel units, one containing a pellet and the other being empty. The half shells adhesively tack together before entering a second cure process step. This second cure permanently fuses the halves together forming pressureless complete ball cores.
U.S. Pat. No. 5,558,325 relates to a hollow rubber play ball that uses a combination of two or more fillers to produce predetermined playing properties in a tennis ball. The preferred ball is a pressureless tennis ball. The specific fillers disclosed are mineral type particulate reinforcing fillers such as silica and carbon black, polymeric fillers such as cellulose, a reaction product of grafting polyethylene with cellulose, and ethylene-vinyl acetate copolymers. These fillers can also have surface treatments to enhance the elastomer-filler interaction. Reinforcing filler that can be introduced with the rubber components is the non-rubber constituents in skim rubber that serve to increase the modulus of a compounded rubber.
U.S. Pat. No. 4,249,730 relates to a tennis ball comprising a hollow sphere of natural rubber and/or synthetic rubber similar in resilience properties to natural rubber, the rubber containing filler comprising chrome-tanned leather particles, and the hollow sphere containing a gas having the same or higher pressure than that of the surrounding atmosphere.
U.S. Pat. No. 4,318,875 relates to a process for the production of a core for a tennis ball consisting of a cross-linked, cellular, elastomeric composition derived from a formulation including (i) at least one elastomeric material selected from the group consisting of natural rubber and synthetic rubbers, (ii) a cross-linking agent, (iii) at least one monomeric salt of an ethylenically-unsaturated carboxylic acid and (iv) a blowing agent, wherein the process includes the steps of (a) thoroughly mixing the elastomeric material and the salt without the application of heat thereto; (b) transferring the mixture of step (a) to a cooled mixer and dispersing therein the cross-linking agent and the blowing agent; (c) extruding the mixture of step (b) through a die and cutting the resulting extrudate into slugs; (d) molding each slug in a pre-heated mold and maintaining the mold at the temperature of pre-heating until decomposition of the blowing agent occurs; (e) transferring each molding of step (d) to a second preheated mold and maintaining the second mold at the temperature of preheating until completion of the cross-linking occurs.
U.S. Pat. No. 4,144,297 relates to a method of forming a baseball or softball core comprising forming a flowable homogeneous mass from a thermoplastic resin such as ethylene vinyl acetate copolymer and a blowing agent by the application of heat and pressure thereto in an injection molding machine; injecting the mass into the spherical cavity of a mold cooled to 32° to 65° F. (0 to 18.3° C.) to foam the mass and fill the mold cavity; holding the mass in the mold cavity for a sufficient time to form a non-porous skin on the outer surface of the mass, the skin being of adequate strength to prevent rupture of the skin by blowing agent contained within the mass upon removal of the mass from the mold cavity; removing the mass from the mold; and cooling the mass at an exterior temperature of 32° to 65° F. (0 to 18.3° C.) to produce a game ball core having a dimension substantially the same as the mold cavity.
U.S. Pat. No. 3,976,295 relates to a composite baseball having the appearance, physical characteristics and dimensions of a conventional baseball, the composite baseball comprising a spherical core member formed of flexible and resilient molded polyurethane foam and a cover portion comprising leather stitched over and enclosing the core member wherein the core member is formed of polyurethane foam of such density and resilience as to give the composite ball essentially the same rebound qualities, hardness, size, feel and sound as a conventional baseball.
Korean patent publication KR 2001-0002975 relates to manufacture of a non-pressurized tennis ball having slower speed and lower bound compared to a ball which would meet ITF specifications. The bound (or rebound) of the tennis ball taught by this patent publication is from 110 to 120 cm when dropped from 254 cm onto horizontal, level concrete. The core of this tennis ball comprises a synthetic resin of either polyurethane or ethylene vinyl acetate foam. The core is manufactured in a cumbersome multi-step process to form a spherical ball. Felt is then glued onto the spherical ball to yield a tennis ball with reduced speed and bound for use as a practice ball by beginners, etc.
The techniques and products of the above patent publications are limited in that they either do not teach or suggest a cellular elastomeric polyurethane foam spherical ball, or if they do suggest such a ball, it is manufactured by a commercially difficult or complicated process or it does not provide a core for a tennis ball which satisfies the strict ITF specifications. None of these references teach or suggest a simple, minimal step method for manufacturing a cellular elastomeric polyurethane foam spherical ball which, when used as a core for a tennis ball, enables the tennis ball to meet the ITF specifications for tennis balls, including for example, weight, diameter, bound, forward deformation and return deformation. The ITF specifications for tennis balls include a weight of from 56.70 to 58.47 grams; a diameter of from 65.4 to 68.6 mm; a bound (or rebound) of from 135 to 147 cm when dropped from 254 cm onto horizontal, level concrete; a forward deformation (or deflection) of from 5.59 to 7.37 mm under an 8.17 kg load; and a return deformation (or deflection) of from 8.89 to 10.8 mm at 8.17 kg load on recovery after the ball has been compressed 25.4 mm.
The present invention results in a cellular elastomeric polyurethane foam ball core which, if desired, provides a tennis ball with properties well within the requirements of the ITF specifications for tennis balls. Furthermore, the tennis balls utilizing cores comprising cellular elastomeric polyurethane foam balls manufactured by the present invention are playable for extended periods of time since they do not loose pressure from play or storage.